1. Field of the Invention
A method and apparatus for transporting and mating a superstructure to a substructure jacket that was previously secured to the water bottom at a desired location in a body of water. The superstructure is moved into place on an adjustable-draft cargo vessel. Once in position, a plurality of fast-acting lifting and lowering devices mounted on the cargo vessel, deposit the superstructure in place on the substructure.
2. Discussion of Related Art
In the erection of large structures in a body of water, it is not uncommon for the builder to make all or part of the structure ashore and then to transport the structure to location aboard a cargo vessel such as a barge or ship having adjustable draft capability. For example, the center span of a bridge may be floated into position on a vessel. The span is mounted on a trestle constructed over the deck of the transport vessel so that the span lies well above the fixed anchoring points to which opposite ends of the bridge span will be fastened. On location, the vessel is ballasted to sink until the span is deposited in place over the anchoring points. Alternatively, tidal motion may be used in place of, or in addition to, vessel draft-adjustment by use of ballast-tank flooding.
That method has been extended to construction of offshore platforms that are used for oil exploration and production. For example, U.S. Pat. No. 4,252,469, issued to G. J. Blight et al, on Feb. 24, 1981 describes a "Method and Apparatus for Installation of Integrated Deck Structure and Rapidly Separating Same from Supporting Barge Means". According to the Abstract, an apparatus is taught that provides lateral force-transmitting arch means extending across a vessel passageway which is laterally bounded by upwardly projecting portions of a substructure and bounded on the top by an integrated deck. The means for assembling the deck and the substructure include horizontal and vertical shock-absorbing and damping means as well as alignment means. Means are provided for lowering the deck to the substructure at a first relatively slow rate and then separating the deck from the vessel at a subsequent faster lowering rate.
U.S. Pat. No. 4,729,695, issued Mar. 8, 1988 to Antonio Silvestri, discloses a method entitled: "A Process for the Installation of the Enbloc Superstructure of an Offshore Platform and Equipment for Carrying It Out Practically." The patent discloses a method for installing the deck of an offshore platform to the fixed legs emerging from the water of the lower structure or jacket including loading the whole superstructure on a vertically movable support platform provided on the deck of a semisubmersible raft or barge, completely submerging the raft in the vicinity of the jacket, the stability being maintained by vertical buoyancy tanks on the raft deck, and lifting the support platform above the protruding ends of the jacket legs independently from wave motion from the sea. Subsequently, lift pillars slide, by means of hydropneumatic jacks, within tubular columns provided in the superstructure until the conical ends of the pillars enter corresponding seats in the jacket legs, and, during a moment of smooth seas, the superstructure is lifted to the desired height using the hydropneumatic jacks while at the same time the support platform is lowered and the raft is ballasted.
U.S. Pat. No. 4,930,938, issued Jun. 5, 1990 to P. J. M. Rawstrom et al. teaches an "Offshore Platform Deck/Jacket Mating System and Method." This is a system for mating a preconstructed barge-mounted deck with a previously installed offshore jacket. The system includes at least two primary load transfer units, at least one secondary load transfer unit and a plurality of drop block assemblies. The primary load transfer units are designed to absorb a portion of the weight of the integrated deck as the integrated deck is lowered onto the jacket. The secondary load transfer units are designed to engage after a portion of the weight of the integrated deck has been absorbed by the primary load transfer unit and to assist the primary load transfer units in absorbing an additional portion of the weight of the deck as it continues to be lowered onto the jacket. The drop block assemblies are designed to rapidly disengage the deck from any contact with the unloaded deck supports that remain on the barge.
Certain problems afflict presently-known offshore deck-installation techniques. Typically, the superstructure must be moved from the place of manufacture to a wharf where it is loaded onto a vessel. Loading may be accomplished by means of a crane, an awkward process that adds expense and introduces complications if, indeed, a crane of adequate lifting capacity can be found. Often, larger structures are skidded onto the vessel. Transportation of the integrated deck or superstructure requires use of supporting trestles which complicate construction of the superstructure itself. The consequent relatively higher center of gravity contributes to instability of the transporting vessel. The known methods for disconnecting the deck from the vessel do not adequately take into consideration the abrupt decrease of the draft of the vessel and the consequent increase in freeboard when the mass represented by the superstructure is fully transferred from the vessel to the jacket legs. In some instances, the vessel is ballasted while transfer is taking place but the ballast-water inflow rate has been found to be too slow to be effective. The drop block assembly of the '938 patent provides rapid disconnection but the assemblies are subject to severe stresses that are conducive to mechanical failure of the system.
There is a need for a simple means for loading a superstructure, such as the integrated deck of an offshore platform, onto a cargo vessel such that the load presents a low profile and center of gravity. Jacking means are needed to lift the superstructure into place over the previously installed jacket. It is particularly important that a rapid, controlled breakaway action be provided to compensate for vessel rebound due to an abrupt decrease in vessel draft when the mass of the superstructure is suddenly transferred from the vessel to the preinstalled jacket. After breakaway, the clearance between the superstructure and the vessel must be sufficient to prevent subsequent impactive damage due to sea-swell-induced vessel heave.